Alveolar type I cells are believed to be derived from type 2 cells during normal turnover of the alveolar epithelium and following injury in the adult lung. Little is understood of the molecular mechanisms that regulate expression of the differentiated type I cell phenotype. Aquaporin-5 (AQP5 is a recently characterized water channel protein expressed exclusively by type I cells, but not type or other cells, in the distal lung, making it a useful new marker of the type 1 cell phenotype. The objective of this project is to investigate the molecular programming underlying lung- and AT I cell-specific expression of AQP5 with the long-range objective of understanding mechanisms of normal cell turnover and repair of the alveolar epithelium following lung injury. In Specific Aim 1, the investigator will test the hypothesis that expression of AQP5 is transcriptionally regulated by cis-regulatory elements in the 5'-flanking region of the gene by identifying and characterizing the DNA motifs in the rat AQP5 5-flanking region that mediate its expression in lung-derived and salivary cells. In Specific Aim 2, she will test the hypothesis that the restricted pattern of AQP5 expression is determined by the balance between cis-regulatory enhancer and repressor elements and their interactions with cognate transcription factors that are expressed or activated in a tissue- and cell-specific manner. To accomplish this, the investigator will perform reporter assays in lung and salivary cells using 5' and 3' deletion constructs, DNAse footprinting and EMSA assays. In Specific Aim 3, she will test the hypothesis that type I cell gene expression can be modulated by targeted delivery of genes under control of the AQP5 promoter/enhancer by characterizing the tissue- and cell-specificities of rat AQP5 promoter/enhancers in transgenic mice. Identification of an type 1 cell-specific promoter may allow selective transfer to type I cells of functional genes in order to modulate their properties and protect against injury to the respiratory epithelium.